1. Field of the Invention
The present invention relates to a method and apparatus for forming an offshore structure and, more particularly, for forming an offshore ice structure in an Artic environment that can withstand large lateral loads and provide a working surface for offshore operations year-round.
2. Description of the Prior Art
With increased activity in the Arctic for petroleum exploration and production, the need arises for offshore structures which can provide a dry working surface year-round. The use of retaining structures offshore as artificial islands is well known. However, the art is primarily limited to a discussion of fill materials, such as sand, gravel, silt, slurry, or the like, as the principle support material circumscribed by the retaining walls of the structure. At times, however, these fill materials may not be readily available. This has necessitated, therefore, the development of alternate designs of offshore Arctic structures which do not use the traditional fill materials.
In addition to the problem associated with the availability of fill materials, lateral loading resulting from the movement of large ice floes has posed a major design problem for the present retaining structures. Traditionally, retaining structures are gravity founded, and are, therefore, vulnerable to lateral forces. To resist the large lateral loads exerted by the ice floes, and thereby prevent sliding of the structure along the sea bottom, gravity base structures are generally very large, i.e. several hundred feet in diameter and weighing several hundred thousand tons.
Recognizing these problems, industry has considered several approaches. One such approach calls for the separation of a large ice plate from a pre-existing ice formation. The ice plate is then grounded to provide an island from which offshore activities can be conducted. However, wave action in the summer months rapidly erodes the unprotected ice plate. Therefore, the plate must be sized to compensate for a pre-determined amount of melting and erosion by wave action during the summer so as to provide a design life of at least two winters. However, to survive the summer months, the ice plate would need to be very large initially, perhaps more than 1000 feet in diameter.
Therefore, there exists a need for a more practical retaining structure that can provide a year-round operating surface and can satisfy the environmental factors, lateral loading criteria and seasonal restraints.